Frequency responsive network



Jan. 3, 1967 J. w. BRAULT 3,296,464

FREQUENCY RESPONSIVE NETWORK Original Filed Oct. 21, 1963 s Sheets-Sheet1 auf auf /71 /0 1 I W0 '0 Frequency Frequency b \m 9?; LLWM/ Fig. 3.

lNVENTOR James W. Bruult ATTORNEYS Jan. 3, 1967 J. w. BRAULT 3,296,464

FREQUENCY RESPONSIVE NETWORK Original Filed Oct. 21, 1963 E Sheets-Sheet2 INVENTOR James W. Bruulf BY 772mm Manatflm ATTORNEY? United StatesPatent C) 3,296,464 FREQUENCY RESPONSWE NETWORK James W. Brault,Princeton, N.J., assignor to Princeton Applied Research Corporation, acorporation of New Jersey Original application Get. 21, 1963, Ser. No.317,718, now Patent No. 3,270,213, dated Aug. 30, 1966. Divided and thisapplication Dec. 20, 1965, Ser. No. 536,248 4 Claims. (Cl. 30783.5)

This application is a division of U8. application Serial No. 317,718filed October 21, 1963, now US. Patent No. 3,270,213 granted August 30,1966.

This invention relates generally to frequency responsive networks, andmore particularly to tunable frequency responsive networks employingresistance-reactance circuit elements.

Two common forms of frequency selective band pass or band rejectnetworks using only one type of reactance element, either inductive orcapacitive, are known as Wein bridge or twin-T networks. To adaptfrequency selective circuits of this type for tuning over a range offrequencies, it is necessary that two or more elements be varied inprecise tracking relationship to maintain a desired frequency responsecharacteristic for the network.

It is an object of the present invention to provide an improvedfrequency selective network, using only one type of reactance device,which is tunable over a relatively wide range of frequencies using onlya single variable element, without significant variations in thefrequency response characteristics of the network.

It is another object of this invention to provide an improved frequencyselective network, using only a single type of reactance device, whichmay be tuned over a very wide range of frequencies using only twovariable elements which need not be maintained in close trackingrelation to maintain a desired frequency response characteristic.

A frequency selective network in accordance with the invention includesinput circuit means with resistive and either capacitive or inductivecircuit elements. Impedance isolation means such as a buffer amplifieris coupled between the input circuit means and a summing networkincluding the series connection of a resistive circuit element and areactive circuit element. Utilization circuit means is coupled betweenthe junction of the series connected summing network elements and apoint of reference potential. The isolation means causes a voltage to beapplied across the summing network which is effectively equal to thedifference between the resistive and reactive voltage componentsdeveloped respectively across resistive and reactive elements of theinput circuit.

To provide a relatively sharp frequency response, it is desirable thatthe time constant of the resistive and reactive elements of the inputcircuit be made approximately equal to the time constant of theresistive and reactive components of the summing network. How- 'ever,due to the isolation means between the summing circuit and the elementsof the input circuit, it has been found that the relationship of thetime constant of the elements of the input circuit means to that of thesumming network is not critical as in prior circuits. As a result, thenetwork can be tuned over a wide range of frequencies by varying onlyone of the elements of the summing network. In tuning over a frequencyrange of about 4 to 1, it was found that the effective figure of meritor Q of the network changed only about i10% and accordingly that thefrequency response characteristic thereof was not materially alteredover the entire frequency range to which the circuit was tuned.

Where the input circuit means includes a pair of time 3,296,464 PatentedJan. 3, 1967 constant circuits each comprising a resistive and areactive device connected in a manner similar to that of the inputcircuit loops of a twin-T network, it is desirable to maintain equaltime constants in the two time constant circuits to provide a relativelysharp frequency response. If a network of this type is to be used totune a frequency range significantly greater than that mentioned aboveand yet maintain the desired bandpass characteristic, it is necessarythat the time constants of each of the time constant circuits of theinput circuit means be adjusted in fairly precise tracking relation asthe time constant of the summing network is varied. On the other hand,the isolation between the summing network and the input circuit meansdoes not require that the time constant of the summing network beprecisely tracked to the time constants of either of the time constantcircuits of the input circuit means.

The isolation means permits a further simplification of the frequencyselective network and a relaxation of the tracking requirements of thetuning elements and at the same time permits tuning of the network overa very wide range of frequencies. A simplified circuit embodying theinvention includes only a single time constant circuit includingresistive and reactive circuit elements across the signal inputterminals to which a signal is applied. The isolation means couples theinput circuit means to the summing network as aforesaid. It was foundthat this circuit could be tuned over a frequency range of the order ofthirty to one without materially changing the frequency response thereofby simultaneous variation of individual elements in the summing andinput circuits. Since the ratio of the time constants of the summingnetwork and the input circuit can be varied over a wide range withoutmaterially altering the frequency response characteristic, precisetracking of these elements is not necessary.

It will be understood that the frequency selective network of theinvention may be used in a wide variety of applications such as, but notexclusive to amplifier circuits and oscillator circuits.

The novel features which are considered to be characteristic of thisinvention are set forth with particularity in the appended claims. Theinvention itself, however, both as to its organization and method ofoperation as well as to additional objects and advantages thereof willbest be understood from the following specification when read inconnection with the accompanying drawings in which:

FIGURE la is a schematic circuit diagram partially in block form of aresistance-capacitance frequency selective network embodying theinvention;

FIGURES lb and 1c are alternate resistance-capacitance networks whichcan be alternatively used in the circuit of FIGURE la to producerespectively the frequency response characteristics shown in FIGURES 3aand 3b;

FIGURE 2 is a graph showing the relationship of the effective Q of thecircuit of FIGURE 1a to the ratio of the time constant of the summingnetwork to the time constant of the input circuit;

FIGURES 3a and 3b are graphs showing the frequency responsecharacteristics which may be obtained with circuit of FIGURE 1a;

FIGURE 4 is a schematic circuit diagram partially in block form of aresistance-capacitance frequency selective circuit in accordance with anembodiment of the invention;

FIGURE 5 is an equivalent circuit diagram representative of the circuitsof FIGURES 1a and 4;

FIGURE 6 is an equivalent circuit diagram illustrating a modification ofthe equivalent circuit diagram of FIGURE 5;

FIGURE 7 is an equivalent circuit diagram representing a simplificationof the equivalent circuit diagram of FIGURE 6;

FIGURE 8 is an equivalent circuit diagram illustrating a modification ofthe equivalent circuit diagram of FIGURE 7;

FIGURE 9 is a schematic circuit diagram partly in block formrepresenting an exemplification of the equivalent circuit diagram ofFIGURE 8; and

FIGURE 10 is a detailed schematic circuit diagram of a tunable frequencyselective amplifier embodying the invention.

Like reference characters will be used to identify like components inthe various figures of the drawings.

Although the circuits described and shown herein comprise onlyresistance-capacitance frequency selective networks it will beunderstood that the underlying principles are also applicable toresistance-inductance networks, and the term reactive element as used inthe specification and claims will be deemed to apply to either acapacitive element or to an inductive element.

The circuit of FIGURE la bears some resemblance to that of a twin orparallel-T network, and may be analyzed in a manner somewhat similar tothat set forth by H. H. Scott, A New Type of Selective Circuit and SomeApplications, Proc. IRE, vol. 26, pp.226235; February 1938, and by W. N.Tuttle, Bridged-T and Parallel-T Null Circuits for Measurements at RadioFrequencies, Proc. IRE, vol. 28, pp. 23-29; January 1940.

Basically the circuit of FIGURE 1a includes an input circuit comprisinga d'ifferentiator R C and an integrator R C coupled to a pair of signalinput terminals 10. The time constants of the diiferentiator and theintegrator are equal. The resultant voltages across the resistor R and Care passed respectively through impedance isolation means 12 and 14which may be buffer amplifiers such as cathode or emitter followershaving a high input impedance and a low output impedance.

The voltage output from the isolation means 12 and 14 are combined in asumming network Z and Z and an output signal may be derived between thejunction of the impedance elements Z and Z and a point of referencepotential shown as ground. Impedance isolation provided by the isolationmeans 12 and 14 provides a new degree of freedom in the design oftunable resistancereactance frequency responsive networks not found intunable passive networks heretofore known in the art.

The open circuit transfer function for the circuit of When it isspecified that T =T =T the transfer function is:

o 1 4j -l- 2 3 e... (1+jwT) Z +Z.)

Considering only resistances and capacitances, there are two choices ofZ and Z which provide circuits of differing frequency response. First,the network Z Z with terminals a, b and c may comprise a network of .thetype shown in FIGURE lb having corresponding ter- Where T3==R3C3.

4 In the special case where K =K =1, the frequency of resonance becomes:

The response characteristics of the network of FIG- URE 1a Where theimpedance elements Z and 2,, are resistive and capacitive respectivelyis shown in FIGURE 3a. From Equation 4 above it can be seen that thefrequency of resonance can be varied by changing T For example, eitherthe resistor R or the capacitor C may be varied to change the resonancefrequency. From Equation 5 above it will be seen that a variation in Talso changes the effective Q and hence the frequency bandpass responseof the network.

With reference to FIGURE 2 which is a plot of network Q as a function ofthe ratio T T it will be seen that T may be varied over an extendedrange of about sixteento-one with a Q variation of only i10%. Hence avariation of T over a range of sixteen-to-one produced only a veryslight change in Q, and, therefore, only a very small change infrequency response as the network is varied over a frequency range ofabout four to one. It is significant that the circuit can be tuned oversuch a frequency range without material changes in bandpasscharacteristic using only a single tuning control element such as theresistor R or the capacitor C The second alternative of the circuit ofFIGURE 1a is that the network Z Z with terminals a, b and 0, maycomprise a network of the type shown in FIGURE 1c having correspondingterminals. In this case the impedance Z comprises a capacitor C and theimpedance element Z comprises -a resistor R As now modified the networkof FIGURE 1a. comprises an active notch filter Whose transfer functionis:

In the special case when K =K =l, the notch frequency is:

And the effective figure of merit or Q of the network is:

Q 2 /TT eff T T4 The response characteristic of the network as nowmodified is shown in FIGURE 3b, and the notch frequency can be varied bychanging T From Equation 9 above it can be seen that T.; can be variedover a relative'ly wide range without causing large changes in Q.Accordingly the network can be tuned over a relatively wide range offrequencies by adjusting only a single control element such as R; or Cwithout causing a material change in frequency response.

In order to increase the frequency range over which the network ofFIGURE 1 may be tuned while maintaining a given tolerance of Qvariation, then T must be varied as well as T or T An importantcondition of the network is that T=T =T accordingly T and T must bevaried in close tracking relation or the transmission of the network atresonance deteriorates rapidly. On the other hand, due to the isolationmeans 12 and 14, there is no stringent requirement that T closely trackT and T The actual extended tuning of the circuit of FIGURE 1 5 Q- inaybe effected by ganging R and R with the resistor in the summing networkfor unicontrol operation, if special care is taken to insure that R andR are adjusted in close tracking relation.

The frequency selective network of FIGURE 4 simplifies the extendedrange tuning problem by eliminating the need for two separate tuningcontrol devices in the input circuit. This network is capable ofcovering a frequency range of the order of thirty to one by varyingsimultaneously only two resistors which need not be kept in precisetracking relation. The input circuit of the FIGURE 4 network includes aseries variable resistor 20 and a shunt capacitor 22. The voltage acrossthe resistor 20 which is the difference between the input voltage at theterminals 24, and the voltage across the capacitor 22, is applied to theunity gain difference amplifier 26. The output voltage of the amplifier26, which provides impedance isolation between its input and outputterminals, thus corresponds to the voltage across the resistor 20.

The voltage across the capacitor 22 is applied to a unity gain bufferamplifier 28. The summing network which includes a variable resistor 39and a capacitor 32 is connected between the difierence amplifier 26 andthe buffer amplifier 28. The total voltage across the summing network isa function of the difference between the voltage across the resistor 20and the voltage across the capacitor 22 and impedance isolation isprovided between the input circuit and the summing circuit by theamplifiers 26 and 28.

An output or utilization circuit is connected to a pair of outputterminals 34 one of which is at ground potential, and the other of whichis connected to the junction of the resistor 30 and the capacitor 32.

The frequency selective network is tuned by the simultaneous adjustmentof the resistors 30 and 2t) which may be ganged for unicontrol operationas indicated by the dashed line 36. Resistors 30 and 20 are adjusted tomaintain the time constant of the resistor 20capacitor 22 networkapproximately equal to the resistor Sit-capacitor 32 network. As notedabove in connection with FIG- URE 3, the ratio of the time constant ofthe summing network to that of the input circuit may vary over arelatively wide range without appreciably altering the Q or thefrequency response characteristic of the network.

The circuit of FIGURE 4 comprises a bandpass network having a frequencyresponse characteristic as shown in FIGURE 2a. If desired, the circuitmay be altered to provide a null response characteristic as shown inFIGURE 2b by interchanging the positions of the variable resistor 30 andthe capacitor 32. Alternatively a null response network may be producedby interchanging the positions of the variable resistor 20 and thecapacitor 22. If desired, a bandpass characteristic may be produced byinterchanging the positions of the variable resistor 30 and thecapacitor 32 as well as the positions of the variable resistor 20 andthe capacitor 22.

The circuits of FIGURES la and 4 may be represented by the equivalentcircuit diagram of FIGURE 5. A voltage source 40 providing a voltage eais connected between a point of reference potential, shown as ground,and one end of the summing network comprising a variable resistor 42 anda capacitor 44 connected in series. A voltage source 46 providing avoltage e is connected between ground and the other end of the summingnetwork. Utilization circuit means is represented by a load resistor 48connected between ground and the junction of the resistor 42 with thecapacitor 44.

The voltage source 40 provides a voltage e equivalent to that developedacross the resistor R of FIGURE 1, or the resistor 20 of FIGURE 4. Inlike manner, the voltage source 46 provides a voltage :2 equivalent tothat developed across the capacitor C of FIGURE 1 or the capacitor 22 ofFIGURE 4.

The equivalent circuit of FIGURE 6 is a modification of that shown inFIGURE 5 wherein the voltage sources 40 and 46 are connected to groundrespectively through voltage sources 50 and 52 each providing a voltagee Voltage e is the same as the voltage e from the source 46 except thatit is shifted in phase by To balance out the effects of the additionalvoltage sources 50 and 52, a voltage source 54 providing a voltage +e isadded to the output voltage from the summing network in an adder circuit56. The adder circuit provides impedance isolation between the voltagesource 54 and the summing network.

Since the voltage sources 46 and 52 provide equal and opposite voltages,they may be eliminated, and the terminal of the capacitor 44 to whichthese sources were connected may be grounded as shown in FIGURE 7. Thecircuit of FIGURE 7 provides an advantage over that of FIGURE 4 in thatparticular care must be exercised in the design of the amplifier 28which drives the capacitor 32, so that the output resistance thereofdoes not adversely affect the time constant of the summing network. Asshown in FIGURE 7, the capacitor 44 is grounded, and only the resistor42 is driven from the input circuit.

The equivalent circuit diagram shown in FIGURE 7 may be redrawn byreplacing the voltage sources 40 and 50 with an equivalent voltagesource 58 which provides a voltage e 2e Note with respect to FIGURE 4that the sum of the voltages across the resistor 20 (e and the capacitor22 (c is equal to the signal input voltage (e Also in FIGURE 1 when T =Tthe sum of the voltages across the resistor R and the capacitor C isequal to the signal input voltages. This relationship is expressed as:

Replacing the voltage source 40 of FIGURE 7 with its equivalent e e asnoted in Equation 11, and combining with the voltage source 50 (e,,)provides a resultant voltage source e 2e as shown in FIGURE 8.

FIGURE 8 is the equivalent circuit diagram of the schematic circuitdiagram, partly in block form shown in FIGURE 9. A signal input voltagee applied to a pair of input terminals 24, is developed across theseries combination of a resistor 20 and capacitor 22 in the same manneras shown in FIGURE 4. The voltage across the capacitor 22 is applied toan amplifier 60 which provides a gain of two and a phase reversal of theinput signal. The output signal from the amplifier 60 ((2e is appliedtogether with the input signal (e to an adder circuit 62. The outputsignal from the adder circuit 62 drives the summing network 42, 44 asdescribed above.

The voltage across the capacitor 22 is also applied to the adder circuit56 previously described in FIGURES 6-8. It should be noted that theadder circuits 56 and 62 not only serve to linearly add the signalsapplied thereto, but also provide impedance isolation between the inputcircuit and the summing circuit.

The circuit of FIGURE 9 provides a bandpass response characteristic ofthe type shown in FIGURE 3a. The frequency of response may be variedover an extremely wide frequency range, of the order of thirty to one,by conjointly varying the resistors 20 and 42 in approximate trackingrelation. The tracking is not critical, and with relatively widevariations between the time constants of the input and summing circuits,the Q or frequency response characteristic of the network remainssubstantially constant over the entire frequency range.

The frequency response characteristic of the network m r c oralternatively as:

may be changed to provide a null by reversing the posi- 1 tions of theresistor and capacitor in either the input or summing circuits. If thepositions of the resistor and capacitor in both the input and summingcircuits are reversed, then the network again exhibits a bandpasscharacteristic.

At this point it should be noted with respect to the figures discussedthus far that resistance-inductance networks may also be used to providewide range tuning. Furthermore, a resistance-capacitance network and aresistance-inductance network can be used respectively in the input andsumming circuits or vice versa, to provide wide range tuning.

A practical exemplification of the block diagram circuit of FIGURE 9 isshown in the schematic circuit diagram of FIGURE 10. The circuit ofFIGURE comprises an amplifier which is tunable over a range of aboutthirty to one. The various transistor stages used in this am plifier areof conventional design, and accordingly a detailed description of allthe circuit components is unnecessary.

A signal input voltage e to be amplified is applied to a pair of inputterminals 24 and is coupled through a capacitor 64 to the base electrodeof a transistor 66. The transistor 66 and a transistor 68 are connectedin cascode relation. The amplified input signal e is developed in thecollector circuit of the transistor 68 and applied to the base electrodeof a transistor 70 which is connected as a phase splitter.

One output signal from the phase splitter is developed across theemitter resistor of the transistor 7 0 and applied to a phase inverterstage including a transistor 72' The signal voltage developed at thecollector electrode of the transistor 72 corresponds to, and is in phasewith the applied signal voltage e,,,, and is applied to the inputcircuit time constant network 74. As mentioned hereinabove withreference to FIGURES 4 and 9 the input circuit time constant networkcomprises a series variable resistor and a shunt capacitor 22.

The voltage developed across the capacitor e is applied to the baseelectrode of a transistor 76. The output signal from the transistor 76is developed at its emitter electrode. The collector electrode of thetransistor 76 is directly connected to the base electrode of atransistor 80. The collector electrode of the transistor 80 is connectedin common to the emitter electrode of the transistor 76 to provide afeedback loop to raise the input impedance and lower the outputimpedance of the transistor 76. The signal at the emitter electrode ofthe transistor 76 is applied to the base electrode of a transistor 78,which is connected as an emitter follower.

The voltage 2 at the emitter electrode of the transistor 78 is appliedto the base electrode of a transistor 82, which provides two times gainfor signals applied to its base electrode. The signal currentcorresponding to e at the collector electrode of the transistor 70 flowsinto the emitter electrode of the transistor 82, and a resultant outputsignal (e -22 is developed at the collector electrode of the transistor82. Thus, the circuit including the transistor 82 corresponds to theamplifier 60 and the adder circuit 62 of FIGURE 9 The resultant voltage(e -22 at the collector electrode of the transistor 82 is applied to thesumming network 84 which includes a variable resistor 42 and capacitor44 connected in series between the collector of transistor 82 and apoint of reference potential which in this case is the negative terminalof the operating potential supply.

The resultant voltage across the capacitor 44 is applied to a transistor86 which is connected as an emitter follower. The output current fromthe collector electrode of transistor 86 provides base current drive fora transistor 88. The collector electrode of the transistor 88 isconnected in common to the emitter electrode of the transistor 86 toprovide a feedback loop to raise the input impedance and lower theoutput impedance of the transistor 86.

The collector electrode of the transistor 90 and the collector electrodeof the transistor 78 are connected in common to provide an adder circuitcorresponding to the adder circuit 56 of FIGURE 9. In order to maintainthe circuit in balance, the gain from the base electrode of thetransistor 86 to the collector electrode of the tran sistor Z matchesthe gain from the base electrode of the transistor 76 to the collectorelectrode of the transistor 92.

The signal output voltage from the adder circuit is taken from thecollector electrode of the transistor 92 and 0s applied to a phasesplitter including a transistor 94. The output terminals from thenetwork comprise either of the terminals 96 or 98 and ground.

A regenerative feedback network is provided from the emitter electrodesof the transistors and 88 to the collector electrode of the transistor66 to enhance the overall frequency response of the network. In thisrespect it will be noted that the transistors 80 and 88 are connected toadd the currents through the emitter resistors of the ransistors '76 and86 so as to produce a resultant feedback signal corresponding to theoutput signal from the network, but isolated therefrom.

The amplifier of FIGURE 10 is tuned by varying the resistors 26 and 42;in rough tracking relation. If desired, the resistors may be ganged forunicontrol operation as indicated by the dashed line 106. The particularband of frequencies over which the amplifier is tunable is a function ofthe time constants of the input and summing circuits. A practicalexample of an amplifier provides a tuning range of 1.5 c.p.s. to kc. infive bands by simultaneously switching different capacitors in place ofthe capacitors 22 and 44 when going from one band to another. The timeconstants of the input and summing networks are kept equal, but asdescribed above considerable latitude in this relationship can betolerated without affecting the Q of the network. As a result, thetroublesome and expensive requirement of maintaining close trackingrelation between the resistors 20 and 42 is eliminated.

Of considerable importance is the fact that circuits embodying theinvention maintain the transmission characteristic of the network atresonance constant as the circuit is tuned over its wide frequencyrange. The reason that this is important in the circuit of FIGURE 10 isthat the feedback loop magnifies any changes in the transmissioncharacteristic of the network and thereby seriously changes the gain andselectivity of the overall circuit.

What is claimed is:

I. A tunable frequency selective network comprising in combination,

input circuit means including a pair of time constant circuits eachcomprising resistive and reactive circuit elements connected in seriesbetween a signal input terminal and a point of reference potential forsaid network,

the time constants of said time constant circuits being equal, I

summing circuit means including resistive and reactive circuit elements,isolation circuit means coupled between said input circuit means andsaid summing circuit means, and

means for adjusting the value of one of said circuit elements of saidsumming circuit means to tune said frequency selective network.

2. A frequency selective network comprising in combination,

means providing an input terminal, and a common terminal,

input circuit means including a first resistive circuit element and afirst reactive circuit element serially connected in the order namedbetween said input and common terminals, and a second reactive circuitelement, and a second resistive circuit element connected in the ordernamed between said input and common terminals, the time constant of saidfirst resistive element and said first reactive element being equal tothe time constant of said second resistive element and said secondreactive element,

first impedance isolation means including an input circuit coupledacross said first reactive circuit element, and an output terminal,

second impedance isolation means including an input circuit coupledacross said second resistive circuit element, and an output terminal,

summing circuit means including a third resistive circuit element and athird reactive circuit element connected in series between the outputterminals of said first and second impedance isolation means, the timeconstant of said summing circuit means being substantially equal to thetime constant of said first resistive circuit element and said firstreactive circuit element,

output circuit means coupled between the junction of said thirdresistive circuit element and said third reactive circuit element andsaid common terminal, and

means for adjusting one of said third resistive circuit elements andthird reactive circuit elements to tune said frequency selectivenetwork.

3. A frequency selective network as defined in claim 2 wherein saidreactive elements comprise capacitors and the third resistive elementand third reactive element are connected in the order named between theoutput terminals of said first and second impedance isolation means.

4. A frequency selective network as defined in claim 2 wherein saidreactive elements comprise capacitors and the third resistive elementand third reactive element are connected in the order named between theoutput terminals of said second and first impedance isolation means.

No references cited.

ARTHUR GAUSS, Primary Examiner.

D. FORRER, Assistant Examiner.

2. A FREQUENCY SELECTIVE NETWORK COMPRISING IN COMBINATION, MEANSPROVIDING AN INPUT TERMINAL, AND A COMMON TERMINAL, INPUT CIRCUIT MEANSINCLUDING A FIRST RESISTIVE CIRCUIT ELEMENT AND A FIRST REACTIVE CIRCUITELEMENT SERIALLY CONNECTED IN THE ORDER NAMED BETWEEN SAID INPUT ANDCOMMON TERMINALS, AND A SECOND REACTIVE CIRCUIT ELEMENT, AND A SECONDRESISTIVE CIRCUIT ELEMENT CONNECTED IN THE ORDER NAMED BETWEEN SAIDINPUT AND COMMON TERMINALS, THE TIME CONSTANT OF SAID FIRST RESISTIVEELEMENT AND SAID FIRST REACTIVE ELEMENT BEING EQUAL TO THE TIME CONSTANTOF SAID SECOND RESISTIVE ELEMENT AND SAID SECOND REACTIVE ELEMENT, FIRSTIMPEDANCE ISOLATION MEANS INCLUDING AN INPUT CIRCUIT COUPLED ACROSS SAIDFIRST REACTIVE CIRCUIT ELEELEMENT, AND AN OUTPUT TERMINAL, SECONDIMPEDANCE ISOLATION MEANS INCLUDING AN INPUT CIRCUIT COUPLED ACROSS SAIDSECOND RESISTIVE CIRCUIT ELEMENT, AND AN OUTPUT TERMINAL, SUMMINGCIRCUIT MEANS INCLUDING A THIRD RESISTIVE CIRCUIT ELEMENT AND A THIRDREACTIVE CIRCUIT ELEMENT CONNECTED IN SERIES BETWEEN THE OUTPUTTERMINALS OF SAID FIRST AND SECOND IMPEDANCE ISOLATION MEANS, THE TIMECONSTANT OF SAID SUMMING CIRCUIT MEANS BEING SUBSTANTIALLY EQUAL TO THETIME CONSTANT OF SAID FIRST RESISTIVE CIRCUIT ELEMENT AND SAID FIRSTREACTIVE CIRCUIT ELEMENT, OUTPUT CIRCUIT MEANS COUPLED BETWEEN THEJUNCTION OF SAID THIRD RESISTIVE CIRCUIT ELEMENT AND SAID THIRD REACTIVECIRCUIT ELEMENT AND SAID COMMON TERMINAL, AND MEANS FOR ADJUSTING ONE OFSAID THIRD RESISTIVE CIRCUIT ELEMENTS AND THIRD REACTIVE CIRCUITELEMENTS TO TUNE SAID FREQUENCY SELECTIVE NETWORK.